Battery sizing
How big a battery do you actually need?
The right battery is the one that gets your house through the evening and night without buying much from the grid, and gets filled by your panels most days. Bigger isn't automatically better, and the 2026 rebate rules make that more true than it used to be.
Start with your night-time use
What matters is how much electricity your home uses between about 6pm and 8am, when the panels aren't producing. Your bill total doesn't show that, but Synergy's My Account lets you download your half-hourly usage. Add up the evening and overnight hours and average them over a few weeks, ideally including summer and winter.
No interval data? As a rough guide, a lot of Perth homes use somewhere between 40% and 60% of their electricity outside daylight hours. Families who are out during the day sit at the higher end.
Turn that into a battery size
- Take your average night-time use, say 15 kWh.
- Add about 15% for charging losses and the reserve the battery keeps back: 15 × 1.15 = 17.3 kWh.
- Divide by the usable capacity of one battery module, and round up.
With SolaX T-BAT modules at 3.3 kWh usable each, 17.3 kWh is six modules (19.8 kWh usable). We usually suggest one spare module on top, because winter evenings run longer and batteries lose a little capacity over the years.
Usable, not nominal
Battery names are based on nominal capacity. Usable capacity is what you can actually draw out, and it's what the federal rebate is calculated on. A SolaX T-BAT HS28.8 is 29.4 kWh nominal and 26.4 kWh usable. If two quotes look like the same size, compare the usable figures.
Where the rebate stops helping
Since 1 May 2026 the federal battery discount pays the full rate on the first 14 kWh of usable capacity, 60% from 14 to 28 kWh, and 15% from 28 to 50 kWh. Capacity up to 14 kWh is heavily subsidised. Beyond it you're paying for more of each extra kWh yourself. That doesn't mean you shouldn't go bigger. It means the extra capacity should be there for a reason: big evening use, an EV, or backup through long outages.
Your panels have to fill it
A battery only saves money if it's full by evening. In Perth, panels produce roughly 4 kWh per kW per day averaged over the year, and noticeably less in June and July. Your system needs to cover your daytime use and charge the battery. A 20 kWh battery on a small array will sit half-empty most of the winter.
Your tariff matters too
With a battery, Synergy's Midday Saver plan is worth a look. As at 1 July 2026 it charges 8.85c per kWh from 9am to 3pm, 24.34c from 9pm to 9am and 55.33c from 3pm to 9pm, with a higher daily supply charge than the standard A1 plan. A battery that covers the 3pm to 9pm window can make it cheaper than A1. Run it against your own usage first, because the peak rate is high if the battery runs flat early.
Want the numbers for your house?
Send us your latest Synergy bill and we'll size a system from your actual usage, show every rebate as its own line, and tell you what we can and can't offer before you sign anything.